I remember when the Cuisinart® food processor first came out. Everybody wanted one. There was even a special magazine filled with recipes that all required a Cuisinart to make. What I came to learn is that for certain things, the Cuisinart is indispensible. For others it is more trouble than it is worth. In fact it makes the job harder. This leads me to the topic of this month’s column: biochar, the solid product of pyrolysis (controlled combustion in a low oxygen environment).
I last wrote about biochar many years ago when it was all the rage, much like the Cuisinart used to be. With time and additional studies, scientists have found that for certain applications biochar can be a terrific material. However, just like the food processor, sometimes there are better tools than biochar for the soil and situation at hand. In the case of biochar, sometimes using it will actually make things worse.
The first benefit attributed to char is soil carbon sequestration over the long term. It is relatively inert, thus not subject to microbial decomposition. When added to soil it typically stays in soil. Recent research has not only confirmed that, it has described in greater detail how adding char to soil can help sequester carbon that isn’t even in the char structure. For example, researchers in Australia added char to soil and came back many years later to find that the char had helped to establish microaggregates in soil where freshly fixed carbon from root exudates can be hidden from hungry microbes (Weng et al., 2017).
This adds to our increasingly sophisticated understanding of carbon storage in soils. It had been understood that carbon that stayed around was in very large molecules like humic and fulvic acids, which had too strong bonds and too big of a size to be readily eaten. Now we are learning that whether the carbon persists in soils is at least partially a function of ease of access — like the ice cream in your hand versus the carton in the basement freezer behind the leftover turkey from last year’s Thanksgiving. One is much easier to gain access to and so more likely to be eaten. If carbon is trapped in small clusters, either protected by biochar or by clay minerals like iron oxides, it is more likely to stay put. So in that way, the carbon in biochar can be very useful. It stays put itself and can provide shelter for additional carbon as well.
That is, unless the char gets crushed. A recent study pointed out that char, while it doesn’t degrade, can lose its structure (Spokas et al., 2014). The authors measured chars produced at different temperatures and from different feedstocks in both samples collected from soils and laboratory-aged materials. They found mass losses of char ranging from 1 to 47 percent. In other words, if you are a microbe taking shelter in a char-built microaggregate, you better call in the structural engineer before making the down payment.

Risks And Benefits

And that is the story pretty much with all of the benefits attributed to biochar. In many cases and with many materials they are realized. But in almost as many cases instead of a benefit, a detrimental impact is observed. Two reviews provide a good summary of the risks and benefits associated with biochar. Jeffrey et al. looked at all of the literature available and saw a mean crop yield increase of about 10 percent. Char produced from poultry litter was overall the best material with a 28 percent yield increase. However yield decreases of up to -28 percent were also observed; char made from municipal biosolids was the worst material tested.
The authors note that char is more likely to benefit acidic soils where it will provide some lime. It is also likely to improve water holding capacity in coarse or sandy soils. Spokas et al. echoes the observation on plant yields with a long list of papers showing a wide range of responses. This review also points out that the general term “biochar” is used to describe materials made with different feedstocks produced using different technologies and having different characteristics. If you start with almond shells and use torrefaction you will end up with a very different material than if you start with poultry litter and use gasification. Total nitrogen in chars is all over the map, ranging from 2 to 53 kilograms per ton of material. Phosphorus and potassium contents are even more varied. We are clearly in need of a char equivalent to the US Composting Council Seal of Testing Assurance protocol.
Some of the mystery associated with biochar can be uncloaked if you look at the feedstocks, the process and your end use goals. Feedstocks used to make char range from materials with very high initial carbon to nitrogen ratios to those that have a surplus of nutrients. If you start with essentially no nutrients, the resulting product will also have no nutrients and is likely to cause nutrient deficiencies if used at high rates and/or in the absence of additional fertilizers. While pyrolysis will result in the loss of much of the nitrogen in a feedstock, it won’t get rid of all of it thus if you start with a material high in nutrients, you will end up with a material that likely has some fertilizer value.
How the char is cooked and treated after the burning and baking also impacts the characteristics of the material. One way to think about biochar is as a building material for soils. Depending on the burn temperature, the char can have very high surface area. Surface tension can also change based on feedstock and burn temperature. In one study higher burn temperatures increased the surface area of chars produced from the same feedstocks (Novak et al., 2012). This surface area can be a benefit or a bust. Materials with high surface area and tension can be excellent for absorbing metals, volatiles, nutrients and even water. The high surface area can increase the ability of the soil to hold onto nutrients and prevent them from leaching.
It can also prevent the nutrients from being available to plants. A study of a hardwood char added to poultry litter during composting found reduced hydrogen sulfide concentrations and losses of N during the composting process (Steiner et al., 2010). Another study found that water holding was increased in weathered soils when switchgrass based chars cooked at both high and low temperatures were added to soils (Novak et al., 2012). Chars made at both high and low temperatures from other materials showed much less benefit.
So where does this leave you? If you live in an area with highly weathered (tropics) or acidic soils, there is a good chance your soils will benefit from biochar. The caveat here is that you have to be sure that there is sufficient fertility either in the char or added alongside it. That is the closest to a sure thing you can get with char. For other applications, the results seem to be varied enough that adding char directly to soils and expecting a result is likely not a good idea. We will probably see niche markets for chars and chars developed for those markets in the near future. Odor reduction and nutrient retention in composting operations is one very promising application. There will certainly be others.
Like the Cusinart, there are likely some excellent uses for these materials. I have a favorite almond cake that I always use the Cuisinart for. When I find that perfect use for biochar I will gladly add it to my soil amendment repertoire. For the moment, I will continue to chop my parsley by hand and use composts and biosolids for my soils.
Sally Brown is a Research Associate Professor at the University of Washington in Seattle and a member of BioCycle’s Editorial Board.